Analysis of specificitis of naturally occurring antibodies in sera from healthy humans, patients with malignancies, and healthy gibbon apes against surface antigens of type-C viruses from primates. Model experiments of immunotherapy of type-C virus-induced spontaneous and primary tumors using cryosurgery or Streptococcus pyogenes. Purification of enhancing and suppressing factors of sarcoma virus-induced primary tumors from blood platelets.